1. Field of the Invention
The present invention relates to a method for forming a corrosion-resistant film for use in a high-temperature apparatus member which is used at high temperatures, such as an industrial gas turbine, a jet engine, a micro gas turbine, an engine, a heat exchanger or a combustor, and also to a high-temperature apparatus member that uses the corrosion-resistant film.
2. Description of the Related Art
High-temperature apparatus members, such as an industrial gas turbine blade and a boiler tube, often have a surface coating in order to enhance the heat resistance and the corrosion resistance. Ceramic coating, called thermal barrier coating (TBC), is generally employed to enhance heat resistance. On the other hand, a Cr or Al diffusion/penetration treatment, thermal spraying of a high Ni-high Cr alloy, or other treatments are employed to enhance corrosion resistance. Thermal spraying, because of its high film-forming rate, is suited for forming a film having a relatively large thickness (not less than 100 μm). However, because of restrictions on the shape and size of a spray gun, thermal spraying is not suited for forming a thin film or forming a film on an article having a complicated shape.
On the other hand, the Cr or Al diffusion treatment to form a protective layer is a gas-phase diffusion method, and therefore is applicable to a fairly complicated article. As shown in FIG. 13, conventional Al diffusion/penetration treatment is carried out by burying a substrate in a mixed powder of Al or Al+M (M is Ni, Co or Cr) with Al2O3, and heating the system at 700 to 900° C. in a hydrogen or inert gas atmosphere. When the substrate is made of a Ni-based alloy, a Ni—Al compound is formed on the surface. The composition of the Ni—Al compound varies depending on the Al concentration of the mixed powder.
Such an Al-rich surface protective layer thus formed, when used in a high-temperature apparatus member, will form an Al2O3 film in the surface at a high temperature. While the Al2O3 film functions as a good protective film because of its low substance diffusion velocity, the film has the drawback that it is likely to peel off e.g. by thermal stress. When the formation of such an Al2O3 film is expected, therefore, it is necessary to use a technique for preventing peel off of the film. In this regard, addition of a small amount of an active element, such as Zr, Hf, Ce, La or Y, to an Al-rich protective layer is known to be effective for preventing peel off of an Al2O3 film from the protective layer. The addition of such an active element is practiced by a physical method, such as thermal spraying or physical vapor deposition. With such a physical method, however, the addition is sometimes difficult for a member having a complicated shape.
The present inventors conducted an experiment, in which a high-temperature member of Ni-20 mass % Cr-10 mass % Mo-18 mass % Fe alloy was subjected to an Al diffusion treatment by the conventional method, and the thus-treated member was exposed to heat at 1000° C. for over 1000 hours. As a result, not only peel off of a surface Al2O3 film was observed, but peel off of a protective layer itself was also observed. Cross-sectional observation of the peel-off portion and its vicinity of the protective layer revealed the formation of a Cr(Mo)-rich layer at the interface between the protective layer and the substrate as well as the formation of a large number of voids at the interface between the Cr(Mo)-rich layer and the substrate, as shown in FIG. 14.
The formation of the Cr(Mo)-rich layer can be explained schematically by using the Ni—Cr—Al ternary phase diagram shown in FIG. 15. When Al diffuses from Ni-50 at % Al toward Ni-about 35 at % Cr (Mo, Fe) (part of Cr may be substituted with Mo and Fe), a two-phase structure, consisting of an α-Cr phase and a γ-NiCr phase, comes to be formed. The Cr(Mo)-rich layer is thus formed at the interface between the substrate and the Ni—Al protective layer. It is considered that as the Cr(Mo) layer grows, voids form due to differences in diffusion velocity between the elements, and the voids will cause peel off of the protective layer.
In addition to prevention of peel off of a Ni—Al coating layer per se, it is also important for extension of the life of a member to secure the adhesion of an Al2O3 film to be formed in the surface of the coating layer.
Such peel off of a protective layer is determined by the composition of a substrate alloy and the diffusion conditions of Al. It is, however, desirable not to change the Al diffusion conditions e.g. from the viewpoint of film-forming rate.